Grinding performances of a sintered alumina abrasive grain are generally enhanced with a decrease of crystal size and an increase of hardness thereof. It is known that, to make microcrystals, magnesium oxide as a sintering aid is added into alumina whereby a magnesium spinel is formed at the crystalline grain boundaries. Further, in Japanese Unexamined Patent Publication No. 56-32369, it has been proposed that, in a process for making an abrasive grain by a sol-gel method, cobalt, hafnium, magnesium, nickel, zinc, zirconium or other metal is added to alumina prior to the step of a gel formation to yield a sintered alumina abrasive grain having a microcrystalline structure.
In Japanese Unexamined Patent Publication No. 60-231462, it has been proposed to dispersing finely divided alpha-alumina seeds in an alumina sol whereby a microcrystalline structure having a crystal size of a submicron order is obtained. In this publication, it is taught that an oxide of silicon, chromium, magnesium or zirconium may be added to an alumina gel.
In Japanese Unexamined Patent Publication No. 61-254,685, it has been proposed that, in a process for making an alumina abrasive grain by a sol-gel method, a nucleating agent such as alpha-alumina or alpha-ferric oxide is added to alumina prior to the drying step. It is taught in this publication that a precursor of an oxide of magnesium, zinc, cobalt, nickel, zirconium, hafnium, chromium or titanium may be added into an alumina gel dispersion.
In Japanese Unexamined Patent Publication No. 64-11184, it has been proposed that, in a process for making an alumina abrasive grain by a sol-gel method, an oxide of zirconium, hafnium, cobalt, nickel, zinc, magnesium and/or a rare earth metal element, or a precursor of the oxide is impregnated in a calcined alumina grain whereby an abrasive grain characterized in that the added metal element is distributed at a higher concentration on the surface and in the vicinity of the surface than that in the inner portion is obtained.
Sintered alumina abrasive grains having a microcrystalline structure can be produced by the above-proposed processes, but the uniformity of the microcrystalline structure and the hardness are not completely satisfactory. Therefore, there is an eager desire of abrasive grains having a uniform microcrystalline structure and a high hardness, and exhibiting excellent grinding performances.